In recent years, plastic films, sheets, injection-molded products, pipes, extrusion-molded products, and blow-molded products are increasingly used in various industrial fields. In particular, polyethylene resins (ethylene polymers) are widely used because of low cost and light weight, a high level of moldability, stiffness, impact strength, transparency, chemical resistance, and recyclability, and other reasons. In general, polyethylene resins are subjected to molding while kept in a melted state. In many cases, however, ethylene homopolymers have insufficient melt properties such as insufficient fluidity and elongational viscosity, have difficulty in maintaining sufficient moldability, or have an insufficient level of solid state properties such as transparency and stiffness.
Among polyethylene resins, linear low density polyethylene (L-LDPE) obtained by catalytic polymerization of ethylene and α-olefin is known as a high-strength resin. However, it is difficult to provide reliable moldability for L-LDPE alone, and L-LDPE has disadvantages such as low transparency and stiffness. As a measure to compensate for these disadvantages, a high-pressure process low-density polyethylene (HPLD) with high moldability or an ethylene polymer as a modifier with a different molecular weight or a different density has been blended to improve the melt properties or the solid state properties.
Unfortunately, the use of HPLD as a modifier can cause a problem such as a reduction in impact strength although it can improve moldability, and the use of an ethylene polymer with a different molecular weight or density can cause a problem such as insufficient moldability or degradation of transparency and gelation due to a widened distribution of molecular weight or copolymer composition.
The current enforcement of the Law for the Promotion of Sorted Collection and Recycling of Containers and Packages and the current trend toward resource conservation require a reduction in the consumption of raw material resins. From this point of view, there is an increasing demand for a reduction in the thickness of molded products. This demand requires an improvement of impact strength and stiffness (elastic modulus).
Reduction of the density of ethylene polymers is a well-known method for improving mechanical strength such as impact strength. However, this method can also reduce the stiffness (or make the polymers soft) and thus is not preferred. Attempts for the thickness reduction include, for example, the use of a combination of two specific ethylene-α-olefin copolymers with different densities and the use of a three-component blend composition further containing a specific HPLD for further improving moldability and transparency (see Patent Literature 1).
These methods can produce polyethylene resin compositions with high transparency and a good balance between impact strength and stiffness as compared with those of traditional compositions. In these methods, however, a reduction in impact strength is inevitably associated with the HPLD blending, and the blending of three ethylene polymers is considered to be economically disadvantageous in terms of stable supply of constant-quality products at an industrial level as compared with transitional methods.
Other known measures to improve mechanical strength such as impact strength include an attempt to use a blend composition comprising a low-density, low-MFR ethylene polymer produced with a metallocene catalyst and a high-density, high-MFR ethylene polymer produced with a metallocene catalyst (see Patent Literature 2), an attempt to use an ethylene-α-olefin copolymer having what is called a reverse comonomer copolymerization composition distribution in which a larger amount of the α-olefin is copolymerized in higher-molecular-weight component switch the aid of a specific metallocene polymerization catalyst (see Patent Literature 3), and an attempt to use an ethylene-α-olefin copolymer having a multi-peak comonomer copolymerization composition distribution produced with a specific hafnocene polymerization catalyst (see Patent Literature 4).
Although these measures can produce polyethylene resin compositions with a higher level of stiffness, impact strength, and ESCR than traditional compositions, such compositions have insufficient moldability due to their low flowability or elongational viscosity and thus are still required to be blended with HPLD or high-molecular-weight polyethylene, which will be inevitably accompanied by impact strength reduction or poor appearance.
Under these circumstances, there have been continued studies on a metallocene polymerization catalyst that is capable of controlling a long-chain branching structure and useful for the development of an ethylene polymer with both high mechanical strength and good molding properties so that the problems with traditional ethylene copolymers or polyethylene resin compositions can be solved, and there have also been continues studies on ethylene polymers produced using such a catalyst (see Patent Literatures 5 and 6). In Patent Literature 6, a transition metal catalyst comprising a specific cyclopentadienyl compound, which has been recently found by the inventor et al., is proposed as a highly active catalyst for ethylene-α-olefin copolymers with preferred long-chain branching.